To optimise diesel engine combustion, it is necessary to have precise control over the quantities of fuel delivered by the fuel injectors. It is desirable to be able to inject small quantities of fuel across a wide range of fuel pressures. For heavy-duty applications in particular, the fuel injectors must be capable of delivering fuel in small quantities at very high fuel pressures.
Typically, a fuel injector includes an injection nozzle having a nozzle needle which is movable towards and away from a valve needle seating so as to control fuel injection into the engine. The nozzle needle is controlled by means of a nozzle control valve (NCV), including a control valve pin, which controls fuel pressure in a control chamber for the nozzle needle. This is especially desirable for the control of the quantity and timing of split injections, which may be required in order to satisfy proposed emissions legislation requirements.
It is recognised that for existing fuel injector designs, dilation effects in the guide bore for the valve needle and the guide bore for the control valve pin give unacceptably high levels of fuel leakage, particularly at the higher fuel pressures (e.g. of the order of 3000 bar) that are demanded of current fuel injection systems. In addition, the control volumes within the injectors are relatively large and result in the injector being less responsive than required for accurate control of multiple injection events.
One way to address these problems is to miniaturize the valve needle and the control valve pin and to reduce the guide bore dimensions accordingly. These have a marked effect on parasitic leakage losses and response times as the associated control volumes and the component masses are reduced in sympathy. In addition, such smaller components require less force to operate them as their masses and the relating hydraulic forces are significantly reduced, enabling faster performance and/or less actuator force requirements. However, miniaturization leads to manufacturing difficulties with existing injector designs.
It is an object of the invention to provide a three-way control valve, suitable for use in a fuel injector, which alleviates the aforementioned disadvantages.